Method of making a semicrystalline ceramic body



lhnite Wil 91D Wi MAKHNG A SEMKCWWTAELW CERAMHC lhtfiilltit Stanley l0.dtoolltey, Corning, l lfifl, nssignor to Corning gins; Works, beg, I l.a corporatian oi New No Drawing. Application December 15', 1958 burialNo. 7%,23ii

This invention relates to the manufacture of semicrystalline ceramicbodies by the controlled g y sgal lization byheaurcatmeutfinhglasshodics and has tb'f its primary object animprovedmethod of heat treatment which is not dependent upon the presencm glassof a nu clegtingor crystallization promoting agent.

Another object is to provide semicrystalline bodies by such improvedmethod which have high silica contents and unusually high thermalexpansion coefiicients.

The semicrystalline bodies produced by the new method have thermalexpansion coefiicients above 175x10- per C., between and 300 C, and, ifdesired, have a glassy surface layer of substantially lower expansioncoeflicient, self-formed while the body is at an elevated temperature,whereby the resulting compressive surface stress at ordinarytemperatures provides the body with a high modulus of rupture. Suchbodies are particularly suited for tableware because of their inherenthigh strength and smooth glassy surfaces which do not show objectionableknife marks from contacts with knives used for cutting food thereon.They also are useful for conjunction with high expansion metals andalloys such as aluminum, copper, brass and the like.

Broadly the new method according to the invention comprises heattreating a glass body comprising 85-92% SiO 65-15% Na O and/or K 0, 0-8%A1 0 and 06% F, the total SiO Na 0 and K 0 amounting to at least 92%, byheating it between 650 and 1250 C. until its linear thermal expansioncoefiicicnt has increased to above 175 x10" per (3., between 0 and 300C.

The invention is predicated upon my discovery that, when suchcompositions are heat treated as described, finely crystallinecristobalite and/or tridymite are formed as a uniform dispersion in amatrix of the residual glass. In general, cristobalite is the principalcrystalline phase, if the temperature of heat treatment does not exceedabout 900 C. for any substantial time; and tridymite is the principalcrystalline phase, if the temperature of heat treatment is at about 900C. or above for a substantial time. If the temperature of heat treatmentis maintained substantially above 900' C. long enough, say a least 2hours, the principal crystalline phase will be substantially solelytridymite, any cristobalite which may have formed being converted totridymite under these conditions. The expansion coefficients of thesetwo crystalline forms of silica are quite high, that; of cristobalitebeing higher than that of tridymite. When the principal crystallinephase is cristobalite the expansion coeliicient of the semicrystallinebody is greater than 300x10 per C. and when the principal crystallinephase is tridymite its expansion coeificient is between 175x10 and280x10 per C. between 0 and 300 C.

When fluorine is present in the composition it forms crystals of NaFand/or KF at temperatures below about; 900 C. Such crystallizationoccurs when the glass is 2033,57 aterlted Apr. 28, 1960 first cooled andbefore the principal phase has been formed by subsequent heat treatment.Such crystals usually are visible as a slight opalescence in the glassand, after the subsequent crystallization of the principal phase, theyconstitute a minor phase in the semicrystalline body. There is noevidence that such minor phase has an crystallization-promoting effect.

Fluorine, which usually is introduced into the batch as an alkali metalcompound, has as its functions to promote melting of the composition andto aid in providing the semicrystalline body with a self-formed surfaceglaze, the advantages of which have been pointed out above. Suchself-glazing occurs automatically when the body is heated substantiallyabove 900 C. It is believed that the alkali metal fluoride, which hasprevious ly crystallized at lower temperatures throughout the body,redissolves in the glassy matrix at temperatures above 900 C. and thatsuch action in the surface of the body forms an external glaze thereonhaving a lower expansion coefficient than that of the body per se andeffectively submerging the crystalline phase or phases beneath suchglaze. If desired, the same result may be obtained independently of thepresence or absence of fluorine by applying to the surface of the bodyby known enamelling technique a glaze composition having an expansioncoefficient suitably lower than that of the body.

Compositions within the above range which may be utilized in carryingout the invention are illusrtated by the batches of Table I which arestated in parts by weight.

Band 514 525 538 514 525 538 519 N85100: 139 118 9B 55 35 14 NBNO: 16. 516. 5 16. 5 16. 5 16. 5 16. 5

F 92 K20 KNOa. 13 A8103 3. 6 3. 6 3. 6 3. 6 3. 6 3. 6 3. 6

To obtain homogeneous glasses, the batches are melted at l400 C. orhigher for at least 4 hours in pots, crucibles or tanks depending uponthe size of the melt. An oxidizing agent such as NaNO or KNO ispreferably contained in the batch and, if desired, a fining agent suchas As O is also added. The amount of As O remaining in the glass has nomaterial effect upon its fundamental characteristics and, since theresidual amount thereof is practically negligible, it is disregarded infurther computations herein.

The exact compositions of fluorine-containing glasses cannot becalculated with accuracy on the conventional oxide basis from theirbatches because a substantial amount of the fluorine is lost byvolatilization during melting. The exact percentage of fluorineremaining in the glass can be determined by analysis but is customarilystated separately from the oxide composition. Analyses offluorine-containing soda-alumina-silicate glasses of remains in theglass.

ausa The modulus of rupture is measured by supporting individual rods ofthe semicrystalline product, about inch in diameter and 4 inches long,on 2 knife edges spaced 3V2 inches apart and individually loading themon 2 downwardly acting knife edges about inch apart and centrally spacedfrom the lower knife edges until breakage of the rods occurs. To makethe results more comparable, the rods are first abraded by being rolledin a ball mill for minutes with grit silicon carbide. Ordinarily, fiveor more rods are thus tested to obtain the average value which iscalculated in p.s.i. Abraded rods of glass in general, when measured inthis manner, show moduli of rupture ranging from 5,000 to 6,000 p.s.i.

On account of the large amount of time involved in the determination ofthe physical properties of the glasses and the semicrystalline productssome of the properties were not measured; but where the physicalproperties have been measured, those properties are given. Even in thosecases where the properties are not given, how- 20 ever, the examplesrepresent actual compositions which s ioi 82.2 an? 09-2 85.2 g 86.5 werecompounded, melted to glasses, and treated in acj: 0 "i 5 cordance withthe teachings herein set forth; and the F 3 5 resulting products had thecharacteristics of the desired ceramics.

Table 111 Glass Heated Commie No. Expn. Bp. 0. Hr. O. Hr. Expn. Sp.p.s.l. phase x10 Gr. X10 Or.

720 1 820 1 1 s0 9w 2 1,150 5 21,000 me. 3% g 1 31,570 ma. s0 720 2 97s4 230 18.150 me. 720 2 840 a 300 10,520 crlst. 37 2. 2004 680 8 3162.3480 .700 crlst. a7 2. 2004 720 .5 310 2. 3477 11.320 crlst. 37 2.2094 720 2 310 2. 3411 10.700 crlst. s7 2. 2024 720 2 B00 1 310 2.340317.740 crist. 37 2. 2094 720 2 970 4 207 20,000 md. 41 720 a s00 10 2012.3123 me. 48 2.2970 000 2 1,150 4 21s 2. 3221 12.000 me. 70 2. 344 7202 000 s 177 2. 357 0,050 ma. 00 2.323 720 2 000 s 191 2.3 2 10.140 ma.n0 2. 308 720 2 200 s 200 2. 347 11.350 ms. 00 2 310 720 2 000 8 21s 2.334 22.000 ms. 58 2.312 720 2 000 s 223 2. s20 20. 280 mu. 720 2 000 s244 2. 209 14.210 ms. 48 2.282 720 2 000 s 224 2.291 14, 780 me.

The hereinbefore recited range of constituents of the From the data ofTable III, and particularly that of glasses which are suitable for thepractice of this invencomposition N0. 4, it will be seen that, when thetempera- P 15 Cfltlcal 1116 following 5 An ture of heat treatment doesnot substantially exceed 900 2 the Stated amount fieficlency 0f anfflllC. for a substantial time, the expansion coelficient of the metal oxidemakes the glass too v1scous for practical samicrystanine product isabove 300 1 7 per e and mehmgl at 8 g'i which are Compatible wall theprincipal crystalline phase is cristobalite but when the present 0 q g'fi i hand temperature of heat treatment substantially exceeds 900 22 2 gg z g g gf gi g i g is g g as; a: C. for a substantial time theexpansion coetficient of the not crystallize when sub ected to thedescribed heat treat- 60 iifi g g g. 1 ments. While A1 0 is not anessential constituent of P an e pnnclpa crys a me P ase the recitedrange, it has a beneficial eiiect in that it inm ymlte' h 1 fii f hcreases the chemical stability of the glasses and the semiof arge t ermaexpanslon we Clem 0 t E crystalline products. Compositions containing1.5% but W Y ZI C product 0f the heat treatment below 900 not more thanabout f 1 0 are, th f 5 C. 1s caused by the mverslon of cristobahtewhich occurs ferred. More than about 8% A1 0 prevents satisfactory innelghbofhood f 200-2Z5 C. and is accompanied crystallization f theglass. by a sudden decrease in density. A similar but much I bl 111 areshown the expansion ffi i t per smaller change In density is caused bythe inversion of C. between 0 and 300 C, i wh l units (E w'l) tridymitewhich occurs in the neighborhood of 117- and specific gravities (Sp.Gr.) of the glasses of Table II 163 C. Such inversions sometimes causebreakageof together with the expansion coefficients, specific gravities,bodies Conialnmg a large Proportion of coafsely F y f moduli of rupture(p.s.i.) and principal crystalline phase cristobalite and/or tridymite.The semrcrystalhne bodies of their corresponding semicrystallineproducts and the of this invention have an extremely fine and umformrespective heat treatments used in converting the glasses crystallinestructure and are able to withstand such sudthereto.

den expansion or shrinking.

What is claimed is:

l. The method of a scmicrystaliine ceramic body having a high thermalexpansion coeiiicient, which comprises heat treating a glass bodycomprising 85-92% SiO 65-15% at least one of the alkali metal oxides NaO and K 0, 0.8% A1 0 and 0-5% F by weight, the total SlDz-l-NflgO-f-IKgOamounting to at icast 92%, by heating it between 650 C. and 1250" C.until its linear thermal expansion coefficient has increased in above175x10 per 6., between 0 and 300 C.

2. The method of claim 1 in which the glass body is heated between 650C. and ahoni 900 C. untii its expansion coeificient is above 300x10" per0., ae 0- 300" C.

3. The method of claim 1 in which the glass body is heated above 900 C.nntii its expansion coefihcieni is 6 between 175 X 10* and 280x10 per C.at 0-300 C 4. The method of claim 3 in which the glass contains 2-5% F.

References Cited in the file of this patent UNITED STATES PATENTS OTHERREFERENCES Mellor: Comprehensive Treatise on Inorganic and 15Theoretical Chemistry, published 1925, vol. 6, page:

1. THE METHOD OF MAKING A SEMICRYSTALLINE CERAMIC BODY HAVING A HIGHTHERMAL EXPANSION COEFFICIENT, WHICH COMPRISES HEAT TREATING A GLASSBODY COMPRISING 85-92% SIO2, 6.5-15% AT LEAST ONE OF THE ALKALI METALOXIDES NA2O AND K2O, 0.8% AL2O3 AND O-5% F BY WEIGHT, THE TOTALSIO2+NA2O+K2O AMOUNTING TO AT LEAST 92%, BY HEATING IT BETWEEN 650*C.AND 125*C. UNTIL ITS LINEAR THERMAL EXPANSION COEFFICENT HAS INCREASEDTO ABOVE 175X10**7 PER *C., BETWEEN 0* AND 300*C.